1. Field of the Invention
The present invention relates to an electric motor, and in particular, to an improved structure for maintaining the insulation of the windings provided in the stator of an electric motor.
2. Description of the Related Art
As shown in FIG. 5B, generally, the core 2 of the stator 1 of an electric motor is constructed by laminating a plurality of thin magnetic steel sheets 3 punched into a predetermined shape to a required thickness and by laminating stator end plates 4 formed from iron sheets having the same shape as that of the thin magnetic steel sheets 3 but whose thickness is slightly greater than that of the magnetic steel sheets 3, at both ends of the laminate. In the prior art example shown in FIG. 5B, the stator core 2, which is the laminate of the magnetic steel sheets 3 and stator end plates 4, comprises an annular yoke portion 5 and a plurality of toothed portions 6 protruding radially inward from the yoke portion 5, forming a slot portion 7 between each two adjacent toothed portions 6.
When the windings (coil) of the stator core 2 have been formed, rather than using copper wire whose cross-section is circular, if a so-called flat copper wire whose cross-section is rectangular (including square) is used, the space factor of the strands of the coil can be increased. Although, normally, the surface of flat copper wire is coated with an insulating resin such as polyamide-imide, it is difficult to insert the flat copper wire into the narrow slot portion 7 and wind it around the toothed portions 6 of the stator core 2, and the method of fitting the coils of each externally wound toothed portion 6 into each toothed portion 6 in a mutually continuous state is also difficult given that the toothed portions 6 are formed inward in the radial direction, the coil is formed from flat copper wire, and the gap in the slot portion 7 is narrow.
In order to solve this problem, in the invention of an AC motor for automotive use registered as U.S. Pat. No. 2,927,288 (Unexamined Japanese Patent Application No. 11-155270 and U.S. Pat. No. 5,998,903), filed by the applicants of the present invention, the flat copper wire whose length is determined as a multiple of the thickness of the laminated stator core 2 is folded at the central portion, a U-shaped conductive segment (also known as a segment conductor) 8 is formed, two leg portions 8a and 8b in each of the plurality of conductive segments 8 are inserted into the slot portion 7 from the direction of one of the stator end plates 4 so that they extend between two slot portions 7 such that the two slot portions 7 are adjacent to each other in the stator core 2, and the leg portions 8a and 8b of the conductive segment 8 protruding from the slot portion 7 in the other stator end plate 4 are folded so that they fall in the same direction as the circumferential direction by means of a jig or the like. Also, with regard to other sets of conductive segments 8 inserted in other adjacent radial positions in each slot portion 7, the leg portions 8a and 8b of those conductive segments 8 are folded so that they fall in the opposite direction to the circumferential direction. In addition, by bringing into contact and welding those portions of the ends of the leg portions 8a and 8b of the radially adjacent conductive segments 8 from which the insulation coating has been removed, a continuous coil is formed.
When such a winding method is employed, there is the possibility that the conductive segments 8 formed from flat copper wire come into direct contact with the edge of the slot portions 7 of the stator end plates 4 when the leg portions 8a and 8b of the conductive segments 8 are folded, and the coating of the flat copper wire at the edge portions or the like is damaged and the insulation is reduced. As a method for eliminating this problem, as shown in FIG. 5A, a method of integrating a coating 9 of synthetic resin having the same shape as the stator end plates 4 by molding it in the position indicated by the arrow, and as well as forming a smooth curved face on the edges 10 of the coating 9 forming opening portions in the slot portions 7 of the stator core 2, inserting tubular insulation paper 11 having a shape that covers most of the interface of the slot portions 7 to maintain insulation between the conductive segments 8 and the stator end plates 4 and the like has been considered. Further, although not utilizing flat copper wire, there are examples where flanges 12 are formed on the end portions of the tubular insulation paper 11 for protecting the coil, and the flanges 12 are brought into contact and engaged with the edges 10 of the coating 9, thus preventing damage to the insulation coating of the coil by the edges 10.
In a case where the means shown in FIG. 5A are employed, given the increase in cost due to molding the synthetic resin coating 9 on the stator end plates 4, it becomes necessary to investigate the suitability of the synthetic resin used as the coating 9 and the usage of the electric motor. For example, where the electric motor is used as the electric motor portion of an electric compressor, and the compressor portion is used as a refrigerant compressor of an air conditioning system, it is necessary to confirm beforehand that there is no possibility of the synthetic resin material of the coating 9 being contaminated by the various types of refrigerating machine oils, refrigerants, and the like which could possibly be used in the air conditioning system. Not only does the development time of the electric compressor thereby increase, but there is also the problem of an over-expenditure occurring as a result.
Also, as in the stator 1 shown in FIG. 6, if only a normal insulation paper 11, formed from an inexpensive elastic synthetic resin material such as PET (polyethylene terephthalate), polyamide or the like, is used as shown in FIG. 5B, without using the coating 9 formed from a synthetic resin as shown in FIG. 5A, and the insulation paper 11 copes with an HFC type refrigerant such as R1134a or the like generally used in air-conditioning systems. In the case of PET, an oligomer is extracted and flows into the refrigerant, accumulates in expansion valves in the refrigeration cycle of the air-conditioning system or portions, such as a screen mesh attached to driers where passages are narrow, and in the worst case giving rise to blockages of the refrigeration cycle. Also, in the case of polyamide paper, because this has a high absorption, there is a problem in that it reduces the insulation of the windings.
Consequently, as a material of the tubular insulation paper 11 as shown in FIG. 6, although it is necessary to use a material whose durability to refrigeration has already been confirmed, such as PEN (polyethylene naphthalate) or PPS (polyphenylene sulfide). However the elasticity of these materials is small, and hence, when the leg portions 8a and 8b of the conductive segments 8 formed from flat copper wire are folded, cracks can occur as indicated by the reference numeral 13, whereby the conductive segment 8 is exposed through the insulation paper 11, comes into contact with the stator end plate 4, and the insulation coating of the conductive segment 8 peels off, as a result of which insulation performance is reduced.